Abstract

Magnesium ferrite (MgFe2O4) nanostructures were successfully fabricated by electrospinning method. X-ray diffraction,
FT-IR, scanning electron microscopy, and transmission electron microscopy revealed
that calcination of the as-spun MgFe2O4/poly(vinyl pyrrolidone) (PVP) composite nanofibers at 500–800 °C in air for 2 h resulted
in well-developed spinel MgFe2O4nanostuctures. The crystal structure and morphology of the nanofibers were influenced
by the calcination temperature. Crystallite size of the nanoparticles contained in
nanofibers increased from 15 ± 4 to 24 ± 3 nm when calcination temperature was increased
from 500 to 800 °C. Room temperature magnetization results showed a ferromagnetic
behavior of the calcined MgFe2O4/PVP composite nanofibers, having their specific saturation magnetization (Ms) values of 17.0, 20.7, 25.7, and 31.1 emu/g at 10 Oe for the samples calcined at
500, 600, 700, and 800 °C, respectively. It is found that the increase in the tendency
ofMsis consistent with the enhancement of crystallinity, and the values ofMsfor the MgFe2O4samples were observed to increase with increasing crystallite size.

Keywords:

Introduction

Spinel ferrites with the general formula AFe2O4 (A = Mn, Co, Ni, Mg, or Zn) are very important magnetic materials because of their
interesting magnetic and electrical properties with chemical and thermal stabilities
[1]. Magnesium ferrite (MgFe2O4) is one of the most important ferrites. It has a cubic structure of normal spinel-type
and is a soft magnetic n-type semiconducting material, which finds a number of applications in heterogeneous
catalysis, adsorption, sensors, and in magnetic technologies [2]. Recently, nanostructures of magnetic materials have received more and more attention
due to their novel material properties that are significantly different from those
of their bulk counterparts [3-7]. The ordered magnetic materials such as nanorods and nanowires have currently attracted
a great interest due to their enhanced magnetic property [8,9]. So far, reported nanostructures MgFe2O4 are mostly in the form of nanoparticle [10-22], whereas other nanostructured forms of MgFe2O4 have not been reported. Large surface-to-volume ratio is an attractive characteristic
that can be achieved from nanofiberization of magnetic materials. With such feature,
their technological application should be expressed into many areas including nanocomposites,
nanocatalysts, nanosensors, nano-electronics, and photonics.

A number of methods have been developed to fabricate materials with nanofibrous structures,
including an electrospinning which is a simple and convenient method for preparing
polymer fibers and ceramic fibers with both solid and hollow interiors that are exceptionally
long in length, uniform in diameter ranging from tens of nanometers to several micrometers,
and diversified in compositions [23,24]. In an electrospinning process [25], an electrical potential is applied between a droplet of a polymer solution held
at the end of the nozzle of the spinneret and a ground target. When the applied electric
field overcomes the surface tension of the droplet, a charged jet of polymer solution
is ejected. The route of the charged jet is controlled by the electric field. The
jet exhibits bending instabilities caused by repulsive forces between the charges
carried with the jet. The jet extends through spiralling loops. As the loops increase
in diameter the jet grows longer and thinner until it solidifies or is collected on
the target.

To date, electrospun ferrite nanofibers of NiFe2O4[26], CoFe2O4[27], MnFe2O4[28], and CuFe2O4[29] have been reported. To the best of our knowledge, electrospinning of MgFe2O4 has not yet been reported. Thus, the present work investigated the fabrication of
MgFe2O4 nanofibers by electrospinning using a solution that contained poly(vinyl pyrrolidone)
(PVP) and cheap Mg and Fe nitrates as metal sources. The samples of as-spun and calcined
MgFe2O4/PVP composite were characterized by thermogravimetric-differential thermal analysis
(TG-DTA), X-ray diffraction (XRD), FT-IR, scanning electron microscopy (SEM), and
transmission electron microscopy (TEM). The magnetic properties of calcined MgFe2O4/PVP composite samples were investigated using a vibrating sample magnetometer (VSM)
at room temperature. The effects of calcination temperature on morphology, structure,
and magnetic properties of the fabricated samples were also studied.

Experimental Section

In this study, Mg(NO3)2 · 6H2O (99% purity, Kanto Chemicals, Japan), Fe(NO3)3 · 9H2O (99.99% purity, Kanto Chemicals, Japan) and PVP (Mn = 1,300,000, Aldrich),N,N-Dimetylformamide (DMF) (99.8% purity, Fluka, Switzerland), acetic acid (100% purity,
BDH, England), and ethanol (100% purity, BDH, England) were used as the starting chemicals.
In the preparation of the solution for electrospinning, we used a solution that contained
PVP mixed with Mg(NO3)2 · 6H2O and Fe(NO3)3 · 9H2O. A PVP/ethanol solution was prepared using a ratio of 1.0 g PVP to 9 mL ethanol.
A metal nitrates/DMF solution was prepared by dissolving 0.01 mol Mg(NO3)26H2O and 0.02 mol Fe(NO3)3 · 9H2O in 10 mL of DMF and stirred for 5 h. Subsequently, the metal nitrates/DMF solution
(4 mL) was added slowly to the PVP/ethanol solution (50 mL) under vigorous stir at
27 °C for 5 h to obtain a well-dissolved solution. This final solution was used for
electrospinning.

The prepared polymer solution was loaded into a plastic syringe equipped with a 22-gauge
needle made of stainless steel. The electrospinning process was carried out using
our home-made electrospinning system. The electrospinning system and schematic diagram
of electrospinning process are shown in Fig. 1. The needle was connected to a high-voltage supply and for each solution the voltage
of 15 kV was applied. The solution was fed at a rate of 0.5 mL/h using a motor syringe
pump. A piece of flat aluminum foil was placed 15 cm below the tip of the needle,
and used to collect the nanofibers. All electrospinning processes were carried out
at room temperature.

Figure 1. An electrospinning system (left) and schematic diagram of electrospinning set up (right)

The as-spun MgFe2O4/PVP composite nanofibers were subjected to TG-DTA using Pyris Diamond TG/DTA (PerkinElmer
Instrument, USA). This was done to determine the temperatures of possible decomposition
and crystallization (or phase changes) of the as-spun nanofibers. The analyses were
performed with a heating rate of 5 °C/min in static air up to 1000 °C. The composite
nanofibers were calcined at 500, 600, 700, and 800 °C for 2 h in air in box furnace
(Lenton Furnaces, UK), using heating and cooling rates of 5 °C/min. The final products
obtained were brown MgFe2O4samples. The as-spun and calcined composite nanofibers were characterized by means
of XRD using CuKa radiation withλ = 0.15418 nm (PW3040 mpd control, The Netherlands), FT-IR spectroscopy (Spectrum
One FT-IR Spectrometer, PerkinElmer Instruments, USA), SEM (Hitachi FE-SEM S–4700,
Japan), and TEM (Philips Tecnai 12 G2 TEM, at 120 kV, The Netherlands). The average
diameters of the as-spun and calcined composite nanofibers were determined from about
300 measurements. The magnetic properties of the calcined samples were examined at
room temperature (20 °C) using a VSM (Lake Shore VSM 7403, USA).

Results and Discussion

The TG curve in Fig. 2 shows a minor weight loss step (~20%) from 30 up to about 270 °C and two major weight
loss steps from 270 to 455 °C (~60%). No further weight loss was observed up to 1000
°C. The minor weight loss was related to the loss of moisture and trapped solvent
(water, ethanol, and carbon dioxide) in the as-spun MgFe2O4/PVP composite nanofibers, whereas the major weight loss was due to the combustion
of organic PVP matrix. On the DTA curve, main exothermic peaks were observed at ~290
and ~450 °C, suggesting the thermal events related to the decomposition of Mg and
Fe nitrates along with the degradation of PVP by dehydration on the polymer side chain,
which was confirmed by a dramatic weight loss in TG curve at the corresponding temperature
range (270–455 °C). The plateau formed between 455 and 1000 °C on the TG curve indicated
the formation of crystalline MgFe2O4 as the decomposition product [30,31], as confirmed by XRD and FT-IR analyses as shown in Figs. 6 and 7, respectively.

Figure 2. TG-DTA curves of thermal decomposition of the as-spun MgFe2O4/PVP composite nanofibers at a heating rate of 5 °C/min in static air

The morphology of the as-spun and calcined MgFe2O4/PVP composite nanofibers was revealed by SEM. Figure 3 shows the SEM micrographs and the respective diameter histogram of the as-spun MgFe2O4/PVP composite nanofibers. The as-spun composite nanofibers appeared quite smooth
due to the amorphous nature of MgFe2O4/PVP composite. Each individual nanofiber was quite uniform in cross section, and
the average diameter of the fibers was 134 ± 35 nm. The PVP was selectively removed
by calcination of the as-spun composite nanofibers in air at 500, 600, 700, and 800
°C. Figure 4 shows the SEM micrographs of the calcined MgFe2O4/PVP composite nanofibers. All the calcined nanofibers formed a structure of packed
particles or crystallites. These changes in the morphology are related to a dramatic
change in crystal structure as observed in electrospun NaCo2O4[30], Ba0.6Sr0.4TiO3[31], and TiO2[32]. The nanofibers calcined at 500 °C remained as continuous structures (Fig. 4a), having fiber size of ~100 nm in diameter. The reduction in size of the nanofibers
should be attributed to the loss of PVP from the nanofibers and the crystallization
of MgFe2O4. After calcination above 500 °C, the nature of nanofibers was changed, and a structure
of packed particles or crystallites was prominent, which may be due to the reorganization
of the MgFe2O4 structure at high temperature. From Fig. 4, the particle sizes of the calcined samples of MgFe2O4/PVP composite nanofibers are <50 nm.

Figure 4. SEM micrographs of the MgFe2O4/PVP composite samples calcined in air at different temperatures for 2 h.a500 °C,b600 °C,c700 °C, andd800 °C

The detailed morphology and crystalline structure of the MgFe2O4/PVP composite nanofibers calcined at 700 and 800 °C for 2 h were further investigated
by TEM, and the TEM bright-field images with corresponding selected-area electron
diffraction (SAED) patterns of these two samples are shown in Fig. 5. It is clearly seen from the TEM bright-field images that both samples consisted
of packed MgFe2O4 particles or crystallites with particle sizes of ~10–20 and 25–80 nm in diameter
for the samples of 700 °C-calcined and 800 °C-calcined composite nanofibers, respectively.
It is seen that the particle sizes of MgFe2O4 contained in the calcined MgFe2O4/PVP composite nanofibers are quite uniform. This might have resulted from the rates
of hydrolysis involved in the fabrication process in which the water required for
the hydrolysis of metal precursors was supplied by the moisture in air [26]. Since the electrospun fibers were very small in diameter, the moisture could quickly
diffuse into the fibers, causing a rapid and uniform hydrolysis of the metal precursors.
The corresponding SAED patterns (Fig. 5) of both samples show spotty ring patterns without any additional diffraction spots
and rings of second phases, revealing their crystalline spinel structure. Measured
interplanar spacings (dhkl) from SAED patterns shown in Fig. 5 are in good agreement with the values in the standard data (JCPDS: 88-1935). The
diffraction rings are identified as the (111), (220), (311), (400), (422), (511),
and (440) planes. This concurs with the results of XRD presented in Fig. 6.

Figure 6. XRD patterns of the MgFe2O4/PVP composite samples calcined in air for 2 h at different temperatures.a500 °C,b600 °C,c700 °C, andd800 °C

The XRD patterns of the calcined MgFe2O4/PVP composite nanofibers are shown in Fig. 6. All of the main peaks are indexed as the spinel MgFe2O4 in the standard data (JCPD no.: 8-1935). The average crystallite sizes of CuFe2O4 samples were calculated from X-ray line broadening of the reflections of (220), (311),
(400), (511), and (440) using Scherrer’s equation (i.e., D = 0.89λ/(β cosθ), where λ is the wavelength of the X-ray radiation, K is a constant taken as 0.89, θ the diffraction angle, and β is the full width at half-maximum [33]), and were found to be 15 ± 4, 17 ± 1, 23 ± 2, and 24 ± 3 nm for the samples of MgFe2O4/PVP composite nanofibers calcined at 500, 600, 700, and 800 °C, respectively. The
values of lattice parameter a calculated from the XRD spectra were 0.8372 ± 0.0007, 0.8362 ± 0.0012, 0.8353 ± 0.0011,
and 0.8346 ± 0.0030 nm for the samples of MgFe2O4/PVP composite nanofibers calcined at 500, 600, 700, and 800 °C, respectively. The
crystallite sizes and lattice parameters are also summarized in Table 1.

Table 1. Average crystal sizes from XRD, spinel lattice parameteracalculated from XRD spectra, the specific magnetization (Ms), remnant magnetization (Mr), the ratio of the ratio of remnant magnetization to bulk saturation magnetization
(Mr/Ms), and coercive forces (Hc) of the MgFe2O4/PVP composite samples calcined in air at 500, 600, 700, and 800 °C for 2 h

The formation of spinel MgFe2O4 structure in the calcined MgFe2O4/PVP composite nanofibers was further supported by FT-IR spectra (Fig. 7). Here, we consider two ranges of the absorption bands: 4000–1000 and 1000–400 cm−1 as suggested by previously published studies [13,34]. In the range of 4000–1000 cm−1, vibrations of CO32− and moisture were observed. The intensive band at ~1627 cm−1 is due to O–H stretching vibration interacting through H bonds. The band at ~2920
cm−1 is C–H asymmetric stretching vibration mode due to the –CH2– groups of the long aliphatic alkyl groups. The ν(C=O) stretching vibration of the
carboxylate group (CO22−) was observed around 1380 cm−1 and the band at ~1016 cm−1 was corresponded to nitrate ion traces. Therefore the CO32− and CO3− vibrations disappeared when calcination temperature was increased. In the range of
1000–400 cm−1, a typical metal–oxygen absorption band for the spinel structure of the ferrite at
~560 cm−1 was observed in the FT-IR spectra of all of the calcined MgFe2O4 samples. This band strongly suggests the intrinsic stretching vibrations of the metal
(Fe ↔ O) at the tetrahedral site [34-37].

Figure 7. FT-IR spectra of the MgFe2O4/PVP composite samples calcined in air for 2 h at different temperatures.aAs-spun,b500 °C,c600 °C,d700 °C, ande800 °C

The specific magnetization curves of the calcined MgFe2O4/PVP composite nanofibers obtained from room temperature VSM measurement are shown
in Fig. 8. These curves are typical for a soft magnetic material and indicate hysteresis ferromagnetism
in the field range of ±500 Oe, while outside this range the specific magnetization
increases with increasing field and tends to saturate in the field range investigated
(±10 kOe). The specific saturation magnetization (Ms) values of 17.0, 20.7, 25.7, and 31.1 emu/g at 10 kOe were observed for the MgFe2O4/PVP composite nanofibers calcined at 500, 600, 700, and 800 °C, respectively. It
is found that the increase in the tendency of Ms is consistent with the enhancement of crystallinity, and the values of Ms for the MgFe2O4 samples were observed to increase with increasing crystallite size. This type of
behavior is entirely consistent with a model of crystal growth in such a way that
the difference in the magnetic parameters is associated with the change in crystallite
size [38]. Noted that the saturation value of 31.1 emu/g obtained in the sample calcined at
800 °C (crystallite size of 24 ± 3 nm) is close to the values of 33.4 emu/g for bulk
MgFe2O4[18] and 30.6 emu/g for sol–gel/combustion synthesized MgFe2O4 (crystallite size of ~78 nm) [13], while it is higher than the values of ~14.09 emu/g for coprecipitation-synthesized
MgFe2O4 nanoparticles (diameters of ~34.4 nm) [21] and 15.3 emu/g for sol–gel-derived MgFe2O4 nanoparticles (diameters of ~42 nm) [22].

From Fig. 8, the remnant magnetization (Mr) values of 0.6, 0.8, 2.4, and 4.7 emu/g were observed for the MgFe2O4/PVP composite nanofibers calcined at 500, 600, 700, and 800 °C, respectively. As
a result, the ratio of remnant magnetization to bulk saturation magnetization, Mr/Ms, of the MgFe2O4/PVP composite nanofibers calcined at 500, 600, 700, and 800 °C was obtained to be
0.035, 0.040, 0.095, and 0.151, respectively. The low values of Mr/Ms indicate an appreciable fraction of superparamagnetic particles. The increase in
Mr/Ms from 0.035 to 0.151 is consistent with results obtained on MgFe2O4 nanoparticles reported by Rashad [21], in which Mr/Ms was increased from 0.113 to 0.137 when particle size increased from 27.2 to 112 nm.
However, our results and those of Ref. [21] are not consistent with results obtained on typical ferromagnetic particles reported
in Ref. [39]. For ferromagnetic nanoparticles, it is interesting to note that the magnetization
is strongly dependent on their particle size, as shown by electron holographic study
of carbon-coated Ni and Co nanoparticles [39]. The ratio of remnant magnetization to bulk saturation magnetization, Mr/Ms, of Co decreased from 53 to 16% and of Ni decreased from 70 to 30% as the particle
diameter increased from 25 to 90 nm. It is clearly seen from this report that the
smaller the particles the higher the remnant magnetization. This is due to the tendency
of smaller particles to be single magnetic domains and larger particles usually contain
multiple domains. The decrease in the Mr/Ms values observed in our samples may be due to an appreciable fraction of superparamagnetic
particles in the samples. However, it is also possible that magnetic anisotropy may
play an important role and further work is needed to achieve thorough understanding.

Figure 8. The specific magnetization of the MgFe2O4/PVP composite samples calcined in air for 2 h at different temperatures, as a function
of field, measured at 20 °C

The coercive forces (Hc) were obtained to be 35.8, 37.6, 71.2, and 98.9 Oe for the MgFe2O4/PVP composite nanofibers calcined at 500, 600, 700, and 800 °C, respectively. These
values are comparable to the values of 48.86–75.99 Oe for coprecipitation-synthesized
MgFe2O4 nanoparticles (diameters of ~27.2–112 nm) [21], but are lower than the value of 165 Oe for sol–gel/combustion-synthesized MgFe2O4 (crystallite size of ~78 nm) [13]. It is seen from our results that the Hc values of the calcined MgFe2O4/PVP composite nanofibers increased with crystallite size. It is known that the variation
of Hc with particle size can be explained on the basis of domain structure, critical diameter,
and the anisotropy of the crystal [39-42]. Rashad [21] reported that Hc increased from 48.86 for 27.2-nm MgFe2O4 nanoparticles to 75.99 for 34.4-nm MgFe2O4 nanoparticles and then decreased to 68.11 Oe for 112-nm MgFe2O4 nanoparticles. In this case, the particle size of the 112-nm MgFe2O4 nanoparticles is possibly larger than that of the critical size and thus results
in the decrease in Hc, while the particle sizes of our electrospun MgFe2O4 samples have not reached their critical size and therefore Hc was increased with increase in crystal size. The values of specific magnetization
at 10 kOe, remnant magnetization (Mr), the ratio of remnant magnetization to bulk saturation magnetization (Mr/Ms), and coercive forces (Hc) are also tabulated in Table 1.

Conclusion

Nanostructures of MgFe2O4have been successfully fabricated using an electrospinning technique. Polycrystalline
MgFe2O4nanostructures (crystallite size of ~15–24 nm) as confirmed by SEAD analysis, XRD
and FT-IR were formed after calcination of the as-spun MgFe2O4/PVP composite nanofibres in air at above 500 °C for 2 h. The calcined samples consisted
of the structure of packed particles or crystallites of <50 nm, as revealed by SEM
and TEM. The crystal structure and morphology of the calcined samples were influenced
by the calcination temperature. All of the electrospun MgFe2O4samples are ferromagnetic, having the specific magnetizations of 17.0, 20.7, 25.7,
and 31.1 emu/g at 10 kOe for the samples calcined at 500, 600, 700, and 800 °C, respectively.
We believe that the electrospun MgFe2O4nanostructures could have potential in some new applications as ferromagnetic nanostructures
for nanocomposites, separation, anodic material in lithium ion batteries, catalysts,
and as electronic material for nanodevices and storage devices.

Acknowledgments

The authors would like to thank the Department of Chemistry, Khon Kaen University
for providing TG-DTA, FT-IR, and VSM facilities, the Science Lab Center, Naresuan
University for providing TEM facilities, the Department of Physics, Faculty of Science,
Ubon Ratchathani University for providing XRD facilities, and the Thai Microelectronics
Center (TMEC) for FE-SEM facilities. This study is supported by The National Nanotechnology
Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its
program of Center of Excellence.